In a Long Term Evolution (LTE) protocol evolution process, a carrier aggregation (CA) technology is introduced in Release (Release, R) 10. This technology allows user equipment (UE) to send or receive data on a plurality of carriers simultaneously. A network device, such as an evolved NodeB (eNB), selects a cell as a primary cell (Pcell) of the UE, and selects a cell corresponding to another carrier as a secondary cell (Scell) of the UE. The UE may send and receive data in a plurality of aggregated cells. Herein, an LTE system that supports CA is referred to as an LTE CA system.
Referring to FIG. 1, in an LTE CA system, each UE is corresponding to a Pcell and one or more Scells. The Pcell is a cell 1 in FIG. 1, and the one or more Scells include a cell 2 and a cell 3 in FIG. 1. Both the primary cell and the secondary cell can be configured to provide a transmission resource for uplink and/or downlink data transmission between a network device and the UE. The UE receives paging in the Pcell, and can perform a contention-based random access process in the Pcell.
Currently, a typical case is that UE in an LTE CA system supports carrier aggregation of a maximum of five downlink carriers and carrier aggregation of a maximum of two uplink carriers. That is, carrier aggregation of five downlink carriers and one uplink carrier is allowed, and carrier aggregation of five downlink carriers and two uplink carriers is also allowed.
FIG. 2 is a schematic diagram of supporting, by UE, carrier aggregation of two uplink carriers and five downlink carriers in a time division duplex (TDD) LTE system. The UE supports two uplink carriers, and therefore, the UE can work in uplink subframes only on two carriers simultaneously, to send sounding reference signals (SRS). In the TDD LTE system, a time division multiplexing duplex mode is used, and channel characteristics of uplink and downlink channels working on a same carrier are similar and reciprocal. Therefore, if the SRSs are sent in the uplink subframes only on the two carriers, channel characteristics of downlink channels only of the two carriers can be obtained, and downlink channel characteristics of remaining three carriers cannot be obtained from the SRSs that are sent in the uplink subframes.